I'm trying to make my Nook (original, latest version) use the entire width of the screen, which is more than the built-in margin selection permits me.

So far I've managed to edit an Epub's CSS to make it have 0 margins. The problem is no matter what font I write in the CSS the "Publisher default" on the device remains Malabar. And as you know once I uncheck the "Publisher default" I also lose the 0 margin I defined.

I've put these in every single style class but the Nook must have a different syntax for fonts.

When you say "original, latest version" are you referring to the Classic or the Simple Touch?
If it is a Simple Touch there are 6 fonts, I think?, available on the device. I think? 3?? on the classic?
Because you can change the CSS I assume it is a non-drm epub.
Here is what I do to all epubs I read and have found it to work most of the time on my Classic, my wife's STR, and on my Kobo.

In Calibre 1. hi-lite the book you wish to convert in your library
2. choose convert books
3. select look and feel
4. about 1/2 way down select the Filter Style Information tab
5. select all 5 boxes.
6. pull slider to bottom and see Other CSS properties
7 In the space add line-height,text-align
When I do this before side loading to the device I am able to change fonts, margins, line spacing, text justification, and font-size on almost all epubs.